Variable stiffness rotor

ABSTRACT

A multibladed rotor having the blades connected to the hub by a hinge mechanism and having a variable stiffness spring extending between the hub and the blade across the hinge mechanism to vary the stiffness of the connection between the blade and the hub.

United States Patent 1 1 3,589,835

72 Inventor Arthur w. Linden [56] References Cited Shelton, Colln-UNITED STATES PATENTS P 842487 2,553,193 5/1951 Hudson @1111 416/135[22] Filed July 17,1969

P d 29 3,228,481 1/1966 E1dred............ 416/131 [451 inane no3,280,918 10/1966 Dreesetal 416/117 [731 H f C 3,310,119 5/1967 Watson416/103(X) m 3,504,989 4/1970 Kisovec 416/141 x) Primary Examiner-Everette A. Powell, Jr. 5 VARIABLE STIFFNESS ROTOR Attorney-Vernon F.HallSChllCl 23 Claims, 15 Drawing Figs. [52] U.S.CI 416/103, ABSTRACT: Amultibladed rotor having the blades con 416/138, 416/140, 416/141,416/149 nected to the hub by a hinge mechanism and having a variable[51] lnt.Cl B64c 27/48 stiffness spring extending between the-hub andthe blade [50] Field of Search 416/ 103, across the hinge mechanism tovary the stiffness of the con- 135, 138, 141, 148, 149 nection betweenthe blade and the hubf PATENTED JUNZQ |97| SHEET 2 OF 4 VARIABLESTIFFNESS ROTOR BACKGROUND OF THE INVENTION 1. Field of Invention I Thisinvention relates to rotors wherein the rotor blades are connected tothe rotor hub through a hinge or other type of pivot connection andwherein a variable stiffness spring extends between the hub and theblades to be actuatable between a first position wherein the rotor hasthe characteristics of a rigid rotor and a second position wherein therotor has the characteristics of a nonrigid rotor, that is a flapping,articulated or flexing rotor and intermediate rotor stiffness positionstherebetween.

2. Description of the Prior Art In the rotor prior art, rotors have beenconventionally fabricated so as to be articulated as shown in U.S. Pat.Nos. 2,815,821 and 2,418,030 or to be rigid as shown in U.S. Pat. Nos.3,106,964 and 2,653,670. While certain rotor constructions use springmembers to connect the blade to the hub to thereby form a flexing rotor,such as in US. Pat. No. 2,949,967, these rotors are otherwiseconventional articulated rotors and the springs are of constantasopposed to variable stiffness.

In the past, rotors were designed to be either rigid or articulateddepending upon which type of rotor best suited most performance regimesof the aircraft and it was understood that compromise performance. wouldbe encountered in the regimes where the rotor design selected isinferior.

In accordance with the present invention, the variable stiffness springis formed by stacking a plurality of laminates, such Experience hasshown that in many fields there are requirements for stiff rotors andfor highly flexible rotors and a given rotor will provide betterperformance if operating as a rigid rotor during certain operatingregimes and as a flexible or articulated rotor during other operatingregimes. It is therefore desirable to have a rotor which can operateboth as a highly stiff rotor and as a highly flexible rotor.

For example, in convertiplanes which include rotors which tilt betweenhelicopter positions and propeller positions, a flexible or articulatedrotor is desirable in the helicopter mode and during the transition tothe propeller mode to reduceblade root moments developed duringtransient maneuvers and different loading conditions. However, in thepropeller mode the airflow is axial and a rigid rotor design is moredesirable.

In addition, in helicopter rotors per se, there are particular flightregimes in which rigid rotors are considered to be superior toarticulated rotors and vice versa. For example, a rigid rotor candevelop control moments without the large angles of tilt of the tip pathplane required by an articulated rotor. This effects the rotor fuselageclearance required in the aircraft design. In a compound type ofvehicle, the rigid rotor can provide aircraft control moments eventhough it supports only a minimum percentage of the aircraft grossweight. This is unlike an articulated rotor which requires lift on therotor to develop control moments.

Still considering a helicopter rotor per se, an articulated rotor isdeemed to be superior to a rigid rotor during forward flight where theblade flapping is used to relieve blade root moments. 7

In other rotors, such as fluid pumps, turbine and compressor rotors andso forth, a requirement for rigid rotor and flexible rotor operation mayexist at different times during the operation of a single rotor and theteachings of this invention would be equally applicable thereto.

SUMMARY OF INVENTION A primary object of the present invention is toprovide a bladed rotor of variable stiffness.

In accordance with the present invention, a rotor which has bladespivotally or otherwise connected to the hub in articulated form,includes a variable stiffness spring extending between the blades andthe hub across the connection and the spring is adjustable to vary thestiffness of the spring and hence the rotor.

as spring steel, and positioning the laminate stack in a first positionwherein they present maximum stiffness to bending and a second positionwherein they present minimum stiffness to bending and intermediatepositions therebetween for intermediate degrees of stiffness. By properselection of the number and characteristics of the laminates, thestiffness ratio of the spring between its first and second positions canbe accurately controlled. I

In accordance with a further aspect of the present invention, thecross-sectional shape of the laminate pack or stack can be of anydesired shape such as square, rectangular in any direction, octangular,oval and circular and the cross-sectional shape can be chosen, withoutcompromising the spring characteristics, to accommodate environmentalphysical constraint problems such as a clearance problem, or toaccommodate a physical strength problem in a mating or coacting part,and for other reasons which will be obvious to those having skill in theart.

The invention provides stiffness control of the rotor so that rotorstifiness can be varied to alleviate problems which are encounteredduring rotor operation, for example, if the rotor is being operated as ahighly stiff or rigid rotor and develops resonance problems, springflexibility and hence rotor flexibility can be changed to alleviate theresonance problem. By way of further example, if the rotor is ahelicopter rotor operating as' a soft or articulated rotor during hoveroperation and aircraft control problems are encountered, thespringstiffness and hence the rotor stiffness can be increased toimprove aircraft control.

In accordance with a further feature of this invention, layers oflow-friction material, such as Teflon, are positioned between adjacentlaminates of the laminate or spring pack.

In accordance with a further feature of this invention, the stiffness ofall the blades of a rotor is varied simultaneously and to the samedegree or the blade stiffness may be varied individually to any desireddegree.

In accordance with another feature of this invention, the variouslaminates of the laminate pack or spring pack are connected atsubstantially their midpoint by any convenient mechanism.

In accordance with another feature of this invention, the variablestiffness spring could be used in a rotor under design and developmentto determine optimum stiffness thereof for utilization in final rotordesign.

Still another feature of the present invention is that the rotorstiffness can be varied in flight.

Still another feature of the present invention is that it can be used torestrain blade flapping and droop.

In accordance with still a further aspect of the invention, the variablestiffness spring is mounted so as to be free of blade centrifugalloading.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a modernconvertiplane shown in the helicopter mode of operation utilizing theinvention.

FIG. .2 is a showing of the FIG. 1 convertiplane in flight in thepropeller mode of operation.

FIG. 3 is a partial showing of the top view of a rotor, partially incross section, utilizing the variable stiffness spring across theflapping hinge of an articulated rotor.

FIG. 4 is a side view of the rotor shown in FIG. 3, partially brokenaway and in cross section for purposes of better illustration.

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 3.

FIG. 6 is a partial showing of a spring or laminate pack to illustratethe construction thereof.

FIG. 7 is a cross-sectional showing of a spring pack of modifiedcross-sectional shape.

FIG. 8 is a side view of an actuator connected to the variawhich aresubstantially apart.

FIG. 9 is a showing of a control mechanism for varying the position of aplurality of spring packs simultaneously and to the same positions anddegree of stiffness.

FIG. 10 is a cross-sectional showing of a spring pack in its maximumstiffness position.

FIG. 11 is a showing of a spring pack in its minimum stiffness position.

FIG. 12 is a cross-sectional of a single laminate from the FIG. 10spring pack in its maximum stiffness position.

FIG. 13 is a cross-sectional of a single laminate of the FIG. 1 1 springpack in its minimum stiffness position.

FIG. 14 is a partial top view of the variable stiffness spring used in asemirigid rotor.

FIG. 15 is a side view of the variable stiffness spring of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, wesee modern convertiplane 10 which includes fuselage 12 which issupported from the runway by landing gear 14 and 16, which may be of thetricycle type. Wings 18 and 20 project laterally from the opposite sidesof fuselage l2 and tail section 22 is positioned at the after end of thefuselage. Tiltable rotors 24 and 26 are positioned at the tips of wings18 and 20 and are shown positioned in FIG. 1, in the helicopter mode ofoperation, where in rotor 24 and blades 28 thereof rotate about axis ofrotation 30 and rotor 26 and blades 32 thereof rotate about axis ofrotation 34. Rotors 24 and 26 may be pivoted or tilted about axes 39 and41 between their F IG. 1 positions and their FIG. 2' propeller mode ofoperation positions, wherein rotor 26 pivots about axis of rotation 38and rotor 24 pivots about axis of rotation 40. Rotors 24 and 26 aredriven by one or more engines 42 and 44 which are connected thereto inany convenient fashion by conventional drive mechanism 46.

Engines 42 and 44 may be of the type more fully described in U. S. Pat.Nos. 2,711,631 and2,747,367, and rotors 24 and 26 may be of the typemore fully described in U.S. Pats. Nos. 2,984,306 and 3,080,927.

It should be borne in mind that while our invention is being disclosedin the environment of a tiltable rotor which is used both as ahelicopter rotor and as an aircraft propeller, it could as well beusedon a helicopter rotor only or on a propeller rotor only, or by anyrotor in which blades are flexible with respect to the hub. Thetilt-rotor convertiplane 10 is used to illustrate an environment of ourinvention because a rotor of minimum stiffness is desired in thehelicopter mode of operation and a rotor of maximum stiffness is desiredin the propeller mode of operation.

Now referring to FIGS. 3-5, we see the invention in greaterparticularity and it should be borne in mind that rotor 26 is preferablyidentical with rotor 24 shown. Rotor 24 consists of rotor hub which ismounted in conventional fashion for rotation about rotor axis ofrotation 30 from rotor drive shaft 54 which projects from and issupported by fuselage 12. Rotor 24 has a plurality of equally spacedprojecting units 56, 58, and projecting therefrom and with eachincluding spaced aligned apertures, such as 62 and 64 therein. I

A plurality of blades 28 project from hub 50 and are mounted forrotation therewith about axis 30 and for pitch change motion withrespect thereto about feathering axis 68 by the action of blade-cuff 70which envelops and supports blade root end 72 through feathering orstack bearings 57. The other end of cuff 70 includes spaced face members74, 67, 76, and 69 which .have aligned apertures 78, 80, 73, and 71therein, and which are in alignment with and concentric withcorresponding aligned apertures 62 and 64 of projections 60 about bladeflapping axis 82. Flapping hinge 61 pivotably connects blade-cuff 28, 70to hub 50. Flapping hinge cylinder 84 extends through aligned apertures62, 64, 71, 73, 78, and so as to pivotally connect cuff 70 and blade 28to blade hub 50 about flapping axis 82. Preferably, bearings of anyconvenient type, such as dry bearings of the carbon ceramic of Teflontype 86 and 88 are positioned between apertures 62 and 64 and bushing 90and 92, which surrounds flapping hinge cylinder 84. It will be notedthat hinge cylinder 84 is separated into two spaced parts 84a and 8417which are located in spaced relation about and along flapping axis 82 toleave aperture 94 therebetween. F lapping hinge sleeve 84a and 841: areretained in position by the coaction of flange ends 75 and 77 coactingwith end nuts 79 and 81.

It will be seen that with this construction, centrifugal loading ofblade 28 will be passed through stack bearing 57 to blade-cuff 70 andthen through flapping hinge 61 to rotor hub 50. In conventional fashion,blade pitch horn 98 connects to blade 28 and is caused to be actuated bya swashplate (not shown) so as to cause blade 28 to pivot aboutfeathering axis 68 with respect to cuff 70 and hub 50, thereby varyingblade pitch either collectively or cyclically or both. It should beborne in mind that while in the construction shown in FIGS. 3 and 4,cuff 70 serves to connect blade root 72 to hub 50, the connection couldbe made direct therebetween through flapping hinge 61 if blade pitchvariation were not desired and could also be made directly between blade66 and hub 50 as in FIG. 14. Blade-cuff 70 should be considered a partof blade 28, however, except for purposes of blade pitch variation.

The construction shown in FIGS. 3 and 4 is an articulated rotor with theblade and cuff unit 66-70 free to pivot or articulate with respect tohub 50 through flapping hinge 61. If a soft spring were caused to extendbetween hub 50 and bladecuff unit 28, 70 the flapping action would bevirtually unimpeded and articulated rotor performance would continue tobe achieved. If, however, a stiff spring were positioned between hub 50and blade-cuff unit 28, 70 there would be very severe resistance toflapping motion and therefore rotor 24 would operate as a rigid rotor.It is accordingly the purpose of this invention to place a variablestiffness spring 100 between hub 50 and blade-cuff unit 28, 70 and thatconstruction will now be described.

The variable stiffness spring 100 comprises a plurality of metal orother material laminates or sheets, such as 102, stacked in side-by-siderelation to form spring or laminate pack 104. As best shown in FIG. 6,spring pack 104 preferably includes a stack-up of layers, sheets orlaminates of metal, such as 102 and 106, with layers 108 and 110 oflow-friction material, such as Teflon, sandwiched therebetween.Preferably, but not necessarily, a connecting mechanism such as bolt andnut unit 112 extends through a slot in all of the layers of spring-pack104 for support purposes. It is essential to the operation of variablestiffness spring 100 that the various laminates at the opposite ends 1l4 and 1 16 of spring pack 104 be free to translate or movelongitudinally with respect to one another, that is, to move withrespect to one another substantially in the direction of feathering axis68. It is also impor tant that the various laminates at the oppositeends 114 and 116 of spring pack 104 be restrained substantially frommoving with respect to one another laterally, that is, in a directionperpendicular to feathering axis 68 and flapping axis 82 when thelaminates are in their FIG. 3 stiff spring position. To permittranslation of the ends of the various laminates at end 1 14 of spring100, stationary cylindrical member 1 18 is supported in and may be apart of hub 50 or may be separate therefrom but connected thereto inconventional fashion. Member 118 is shaped to'receive inner cylindricalmember 120 therewithin so as to permit rotary motion of member 120within member 1 18 concentrically about flapping axis 68.

Conventional dry bearings 122 and 124 are preferably positioned betweenfixed cylinder 118 and rotatable cylinder 120. As best shown in FIG. 5,rotatable cylinder 120 has rectangular aperture 126 therein of selectedshape to receive end 114 of spring pack 104 therein with sufficientlooseness to permit longitudinal movement (in and out of plane of paperin FIG. 5) between adjacent laminates yet restrain lateral movement(left and right in FIG. 5 therebetween. It should be borne in mind thatwhile spring pack 104 is illustrated in FIGS. 3-5 to be substantiallysquare in cross section, the spring pack could be of any selected shapein cross section such as square,

rectangular with its major axis in any selected direction, oval,circular or, as best shown in FIG. 7, of a polygon shape. The oppositeend 116 of spring pack 104 is received in a correspondingly shapedaperture 128 of rotatable ring member 130, which is supported forrotation within cuff 70 and preferably has dry ring bearings 132 and 134between the mating OD flanges of cylindrical member 130 and the ID,female flanges of cuff 70. Cylindrical member 130 is prevented fromtranslating toward blade 28 by a conventional snap ring 136 (see FIG. 3)and is prevented from translating toward axis of rotation 30 by lip 138of fixed cylindrical member 118. It will accordingly be seen that springpack 104 is positioned so as to extend through flapping axis 82 and withits opposite ends supported in hub 50 and blade-cuff 28, 70 so as toform a spring member therebetween when blade-cuff 28, 70 attempts toarticulate or pivot about flapping axis 82.

As best shown in FIGS. 3 and 8, rotatable cylindrical member 120 hashorn or crank member 140 extending laterally therefrom and may beactuated in any conventional fashion to rotate through 90 so as to causespring pack 104 to move from its FIG. 35 stiff spring position to aposition 90 therefrom, which will be the soft spring or articulatedrotor position. One such actuating mechanism is shown in FIG. 8 whereinconnecting rod 142 pivotally connects to crank 140 and is caused toactuate at pilot command by hydraulic pistoncylinder arrangement orelectric motor 144.

In many installations it will be desirable to cause the stiffness ofeach of the blades to be equal and to vary the same amount andaccordingly, as best shown in FIG. 9, it may be desirable to connect thevarious spring packs 104 through their individual links 142 to star orplate member 146, which is in turn caused to translate along axis 30 byone or more prime movers, such as electric motor or hydraulicpiston-cylinder 144.

While variable stiffness spring 104 is shownin FIGS. 3-5 to extendsubstantially equal distances on opposite sides of flapping hinge 82 andfeatheringhinge 68 and to have its major axis perpendicular to theformer and parallel to the latter, this positioning of the spring 100 isnot essential.

To explain the operation of variable stiffness spring 100, referencewill now be made to FIGS. -13. FIGS. 10 and 12 show spring pack 104 andan individual laminate 102 thereof in their maximum stiff spring orrigid rotor position. FIGS. 11 and 13 show spring pack 104 andindividual laminate 102 thereof in their minimum stiffness orarticulated rotor position, 90 from the FIG. 10-12 position.

It can be shown mathematically that the moment of inertia l of arectangular laminate can be represented by the following equation:

Equation 1 I=l/l2 bh where I is the moment of inertia of the laminate, bis the width of the laminate and h is the height of the laminate.

By viewing FIGS. 12 and 13, it will be noted that the base of thelaminate b in the FIG. 12 construction is very small while the height hof the laminate therein is very large and therefore its moment ofinertia I I will be large. It will also be notedthat in FIG. 13, thebase of the laminate b is very large while the height h thereof is verysmall so that its moment of inertia I is comparatively small. Inpractice, a spring pack consisting of 88 laminates of spring steel .040inch thick and approximately 4 inches wide produces a variable stiffnessspring whereon the spring in its stiff FIG. 10 and 12 positions has amoment of inertia 1, equal to 19.6 in. and wherein the moment of inertial when in the FIG. 11 and 13 position equals 0.00191 infi. Accordingly,the ratio of moments of inertia for spring pack 104 when in its FIG. 10and 12 stiff spring or rigid rotor position compared to its FIG. 11 and13 soft spring or flexible rotor position is 10,300 to 1. In otherwords, spring pack 104 is 10,300 times as stiff in its FIG. 10 positionthan when in its FIG. 11 position, when the spring pack is fabricated asstated above. Accordingly, a bending force which establishes a I inchdeflection in spring 104 in the FIG. 11 position, will establish a0.000] inch deflection in spring 104 in the FIG. 10 position.

As spring pack 104 is rotated to intermediate positions between its FIG.10 and FIG. 11 end positions, the stiffness of the spring varies witheach such position so that spring 100 is actually a variable stiffnessspring which can be varied in stiffness not only to its FIG. 10 maximumstiffness position and to 1 its FIG. 11 minimum stiffness position, butalso to intermediate stiffnesses of the positions therebetween.

It will be evident to those skilled in the art that this variablestiffness spring can be utilized to produce a rotor which is articulatedwhen the spring is in its soft spring or minimum stiffness position andto produce a rotor which is rigid when the spring is in its hard springor maximum stiffness position and that the pilot accordingly is free tochoose between rigid rotor operation and articulated rotor operationduring the modes of operation when each has its particular advantage. lnaddition, this variable stiffness spring could be used to soften thestiffness of an otherwise rigid acting rotor should such rigid rotor"get into resonant vibration. In addition, such a variable stiffnessspring could be used as a test or calibration device during the initialtesting of a rotor in its development stage so as to determine therefromthe stiffness of rotor required for optimum rotor performance.

While the rotor illustrated herein is an articulated rotor withprovisions for articulated motion about a flapping hinge, it should beborne in mind that the invention could as well be used with anarticulated rotor having a lead-lag hinge only or with a rotor havingboth a lead-lag and a flapping hinge with a variable stiffness springbeing used with each hinge.

In addition, such springs could as well be positioned within rotorswhich have no hinges but which are designed to have inherent flexibilityand would serve to vary'the stiffness thereof as well.

Viewing FIGS. 14 and 15 we see variable stiffness spring used insemirigid rotor 24' which includes hub 50 mounted for rotation aboutaxis of rotation 30 and having a plurality of blades 28 projecting fromhub 50 for rotation therewith and supported therefrom through stackbearings 57 to permit pitch change motion therebetween, if desired, andincluding flexible section about which blade 28 bends with respect tohub 50. In such a construction, variable stiffness spring 100, which isof the same construction as shown in FIGS. 2-5, is supported above blade28' by bearing members 152 and 154 to be pilot actuated through link 156to ring 158 and which is caused to rotate by the pilot through theactuation of pilot operated link 160. It will be obvious to thoseskilled in the art that spring member 100 in the FIGS. 14 and 15construction could as well have extended between hub 50 and station 154of blade 28'.

I wish it to be understood that I do not desire to be limited to theexact details of construction shown and described, for obviousmodifications will occur to a person skilled in the art.

I claim:

I. A rotor having:

a. a hub mounted for rotation about an axis of rotation,

b. a plurality of blades projecting from said hub for rotationtherewith,

c. a hinge member connecting each of said blades to said hub so thatsaid blade is capable of articulating motion thereabout with respect tosaid hub,

d. avariable stiffness spring mounted so as to be free of bladecentrifugal loading and extending between said hub and said blade andextending across said hinge member.

2. Apparatus according to claim 1 wherein said variable stiffness springincludes a plurality of laminates of selected dimension and positionedin stacked relation to form a spring pack and with the ends thereof freeto move longitudinally relative to one another.

3. Apparatus according to claim 2 wherein said variable stiffness springcomprises a plurality of metal laminates of selected height and widthstacked with respect to one another to form a spring pack and so thatthe ends thereof are positioned on opposite sides of the axis of thehinge member and are free to move relative to one another toward or awayfrom the hinge axis.

4. Apparatus according to claim 3 and including means to move saidspring pack to a first position wherein each laminate presents a crosssection of maximum stiffness and to a second position wherein eachlaminate presents a cross section of minimum stiffness.

5. Apparatus according to claim 4 wherein in said first position saidlaminates individually present a cross section of greater height thanwidth, and wherein, in said second position, said laminates individuallypresent a cross section of greater width than height to thereby vary thestiffness of the spring pack. v

6. Apparatus according to claim 2 wherein said spring pack is ofrectangular cross section.

7. Apparatus according to claim 2 wherein said spring pack is of crosssection other than rectangular.

8. A helicopter rotor having:

a. a hub mounted for rotation about an axis of rotation,

b. a plurality of blades projecting from said hub for rotationtherewith,

. a hinge member connecting said blades to said hub for articulatingmotion with respect to said hub thereabout,

. a variable stiffness spring member mounted so as to be free of bladecentrifugal loading and including a spring pack comprising a stack oflaminate members stacked in loose relation with respect to one anotherand extending across said hinge member axis,

e. means supporting the first end of said spring pack in said hub sothat said laminates are free to move longitudinally with respect to oneanother but are restrained from moving laterally with respect to oneanother,

means to support the other end of said spring pack in said blade so thatthe ends of the laminates thereof are free to move longitudinally with.respect to one another but are restrained from moving laterally withrespect to one another,

g. means to move said spring pack to a first position wherein saidlaminates present maximum stiffness to bending about the hinge memberaxis so that said rotor is operable as a rigid rotor and to a secondposition wherein said laminates present minimum stiffness to bendingabout said hinge member axis so that said rotor is operable as anarticulated rotor.

9. Apparatus according to claim 8 wherein said actuating means includesmeans to position said spring pack in intermediate positions betweensaid first and second positions.

10. Apparatus according to claim 9 wherein said spring pack consists ofa selected number of metal sheet laminates of selected cross sectiondimension and wherein each of said laminates presents a cross section ofgreater height than width when in said first position and a crosssection of greater width than height when in said second position so asto constitute a spring of variable stiffness.

11. Apparatus according to claim 10 and including a layer oflow-friction material sandwiched between adjacent laminates of saidspring pack.

12. Apparatus according to claim 11 wherein each of said laminates is ofthe same cross-sectional shape and is of the same height and width andof selected number to produce a variable spring of selected stiffnessratio when in said first and said second positions.

13. Apparatus according to claim 12 wherein said laminates are ofdifferent cross-sectional dimension so that the cross secmember is aflapping hin e.

15. Apparatus accor mg to claim 8 wherein each of said laminate membersis identical and is of rectangular shape and of rectangular crosssection and wherein said spring pack is of rectangular cross section.

16. Apparatus according to claim 8 and including means joining saidlaminate members of said spring pack.

17. Apparatus according to claim 8 wherein said variable stiffnessspring member is the same for each of said blades and including:

a. means to cause said variable stiffness spring members to vary instiffness simultaneously and to the same degree.

18. A helicopter rotor having:

a. a hub member mounted for rotation about an axis of rotation,

b. a plurality of blades projecting radially from said hub member andhaving a root end,

c. a blade-cuff supporting said blades for pitch change motion about ablade feathering axis,

d. a hinge member connecting said cuff to said hub so that said bladeand cuff are supported for pivot motion about the hinge member axis fromsaid hub member,

e. a variable stiffness spring mounted so as to be free of bladecentrifugalloading and extending across said hinge member axis andsupported at its opposite ends in said hub member and said cuff.

19. Apparatus according to claim 18 wherein said variablestiffnessjspring includes a plurality of metal sheets of selectedcross-sectional shape and positioned in stacked relation with respect toone another.

20. Apparatus according to claim 19 and including means to support oneend of said sheets in said hub so that said sheets are free to movelongitudinally with respect to one another but are substantiallyrestrained from lateral motion with respect to one another and means tosupport the other end of said sheets in said cuff so that said sheetsare free to move longitudinally with respect to one another but arerestrained from moving laterally with respect to one another and meansto pivot said sheets so stacked between a first position wherein each ofsaid sheets presents a cross section of maximum height and minimum widthwith respect to said hinge axis so as to constitute a spring of maximumstiffness extending across said axis between said hub and said cuff anda second position wherein each of said sheets presents a cross sectionof minimum height and maximum width with respect to said hinge axis sothat said sheets so stacked constitute a spring of minimum stiffnessbetween said hub and said cuff across said hinge axis.

21. Apparatus according to claim 20 and wherein said hinge member is aflapping hinge.

22. Apparatus according to claim 18 and including means to support saidvariable stiffness spring in said hub and said cuff so that bladecentrifugal loading is not imparted thereto but passes through said cuffand hinge member to said hub member.

23. A helicopter rotor having:

a. a hub mounted for rotation about an axis of rotation,

b. a plurality of blades projecting from said hub for rotationtherewith, andhaving a flexible section therein,

c. a variable stiffness spring mounted so as to be free of bladecentrifugal loading and extending across said flexible section andmounted for flexing motion therewith.

1. A rotor having: a. a hub mounted for rotation about an axis ofrotation, b. a plurality of blades projecting from said hub for rotationtherewith, c. a hinge member connecting each of said blades to said hubso that said blade is capable of articulating motion thereabout withrespect to said hub, d. a variable stiffness spring mounted so as to befree of blade centrifugal loading and extending between said hub andsaid blade and extending across said hinge member.
 2. Apparatusaccording to claim 1 wherein said variable stiffness spring includes aplurality of laminates of selected dimension and positioned in stackedrelation to form a spring pack and with the ends thereof free to movelongitudinally relative to one another.
 3. Apparatus according to claim2 wherein said variable stiffness spring comprises a plurality of metallaminates of selected height and width stacked with respect to oneanother to form a spring pack and so that the ends thereof arepositioned on opposite sides of the axis of the hinge member and arefree to move relative to one another toward or away from the hinge axis.4. Apparatus according to claim 3 and including means to move saidspring pack to a first position wherein each laminate presents a crosssection of maximum stiffness and to a second position wherein eachlaminate presents a cross section of minimum stiffness.
 5. Apparatusaccording to claim 4 wherein in said first position said laminatesindividually present a cross section of greater height than width, andwherein, in said second position, said laminates individually present across section of greater width than height to thereby vary the stiffnessof the spring pack.
 6. Apparatus according to claim 2 wherein saidspring pack is of rectangular cross section.
 7. Apparatus according toclaim 2 wherein said spring pack is of cross section other thanrectangular.
 8. A helicopter rotor having: a. a hub mounted for rotationabout an axis of rotation, b. a plurality of blades projecting from saidhub for rotation therewith, c. a hinge member connecting said blades tosaid hub for articulating motion with respect to said hub thereabout, d.a variable stiffness spring member mounted so as to be free of bladecentrifugal loading and including a spring pack comprising a stack oflaminate members stacked in loose relation with respect to one anotherand extending across said hinge member axis, e. means supporting thefirst end of said spring pack in said hub so that said laminates arefree to move longitudinally with respect to one another but arerestrained from moving laterally with respect to one another, f. meansto support the other end of said spring pack in said blade so that theends of the laminates thereof are free to move longitudinally withrespect to one another but are restrained from moving laterally withrespect to one another, g. means to move said spring pack to a firstposition wherein said laminates present maximum stiffness to bendingabout the hinge member axis so that said rotor is operable as a rigidrotor and to a second position wherein said laminates present minimumstiffness to bending about said hinge member axis so that said rotor isoperable as an articulated rotor.
 9. Apparatus according to claim 8wherein said actuating means includes means to position said spring packin intermediate positions between said first and second positions. 10.Apparatus according to claim 9 wherein said spring pack consists of aselected number of metal sheet laminates of selected cross sectiondimension and wherein each of said laminates presents a cross section ofgreater height than width when in said first position and a crosssection of greater width than height when in said second position so asto constitute a spring of variable stiffness.
 11. Apparatus according toclaim 10 and including a layer of low-friction material sandwichedbetween adjacent laminates of said spring pack.
 12. Apparatus accordingto claim 11 wherein each of said laminates is of the samecross-sectional shape and is of the same height and width and ofselected number to produce a variable spring of selected stiffness ratiowhen in said first and said second positions.
 13. Apparatus according toclaim 12 wherein said laminates are of different cross-sectionaldimension so that the cross section of the spring pack is of selectedshape and constitutes a spring of desired stiffness when in said firstposition and to constitute a spring of desired stiffness when in saidsecond position.
 14. Apparatus according to claim 8 wherein said hingemember is a flapping hinge.
 15. Apparatus according to claim 8 whereineach of said laminate members is identical and is of rectangular shapeand of rectangular cross section and wherein said spring pack is ofrectangular cross section.
 16. Apparatus according to claim 8 andincluding means joining said laminate members of said spring pack. 17.Apparatus according to claim 8 wherein said variable stiffness springmember is the same for each of said blades and including: a. means tocause said variable stiffness spring members to vary in stiffnesssimultaneously and to the same degree.
 18. A helicopter rotor having: a.a hub member mounted for rotation about an axis of rotation, b. aplurality of blades projecting radially from said hub member and havinga root end, c. a blade-cuff supporting said blades for pitch changemotion about a blade feathering axis, d. a hinge member connecting saidcuff to said hub so that said blade and cuff are supported for pivotmotion about the hinge member axis from said hub member, e. a variablestiffness spring mounted so as to be free of blade centrifugal loadingand extending across said hinge member axis and supported at itsopposite ends in said hub member and said cuff.
 19. Apparatus accordingto claim 18 wherein said variable stiffness spring includes a pluralityof metal sheets of selected cross-sectional shape and positioned instacked relation with respect to one another.
 20. Apparatus according toclaim 19 and including means to support one end of said sheets in saidhub so that said sheets are free to move longitudinally with respect toone another but are substantially restrained from lateral motion withrespect to one another and means to support the other end of said sheetsin said cuff so that said sheets are free to move longitudinally withrespect to one another but are restrained from moving laterally withrespect to one another and means to pivot said sheets so stacked betweena first position wherein each of said sheets presents a cross section ofmaximum height and minimum width with respect to said hinge axis so asto constitute a spring of maximum stiffness extending across said axisbetween said hub and said cuff and a second position wherein each ofsaid sheets presents a cross section of minimum height and maximum widthwith respect to said hinge axis so that said sheets so stackedconstitute a spring of minimum stiffness between said hub and said cuffacross said hinge axis.
 21. Apparatus according to claim 20 and whereinsaid hinge member is a flapping hinge.
 22. Apparatus according to claim18 and including means to support said variable stiffness spring in saidhub and said cuff so that blade centrifugal loading is not impartedthereto but passes through said cuff and hinge member to said hubmember.
 23. A helicopter rotor having: a. a hub mounted for rotationabout an axis of rotation, b. a plurality of blades projecting from saidhub for rotation therewith, and having a flexible section therein, c. avariable stiffness spring mounted so as to be free of blade centrifugalloading and extending across said flexible section and mounted forflexing motion therewith.